1. Field of the Invention
This invention relates generally to radio frequency amplifiers and, more particularly, to radio frequency amplifiers having feedforward for reduction of distortion.
2. Description of the Related Art
Radio frequency (RF) amplifiers are used in a wide variety of applications, including communications. Ideally the transfer function of an RF amplifier is linear, with the output of the amplifier being an amplified replica of the input to the amplifier. However, RF amplifiers typically have some degree of non-linearity in their transfer function, particularly at high power levels. This non-linearity in an RF amplifier produces distortion in the RF amplifier output.
One approach used to reduce the distortion during RF power amplification is the feedforward amplifier. In a feedforward amplifier the instantaneous difference between a sample of the amplifier input signal and a sample of the amplifier output signal is measured. The difference signal is then amplified and subtracted from the RF power amplifier output, the amount of amplification being such as to cancel the existing distortion of the RF power amplifier.
FIG. 1 is a block diagram of a conventional feedforward amplifier 100. An input signal is coupled to an input directional coupler 101. The input directional coupler 101 divides the input signal sending a main portion of the input signal to a main amplifier 102 via a first loop modulator 103, and a sample, or sense, portion to a first loop delay 104. The main amplifier 102 amplifies the portion of the input signal routed to it. Amplification of the input signal by the main amplifier 102 produces distortion in the signal output from the main amplifier 102. The output of the main amplifier 102 is routed through a first loop sampling directional coupler 106 to a second loop delay 108.
The first loop directional coupler 106 extracts a sample of the output of the main amplifier 102 and routes the sample to a power combiner 110. The other input to the power combiner 110 is the sense portion of the input signal output from the first loop delay 104. The delay the sense portion of the input signal experiences as it passes through the first loop delay 104 is selected to match the delay experienced by the main portion of the input signal as it passes through the first loop modulator, the main amplifier 102 and the distortion sampling directional coupled 106. The power combiner 110 destructively sums the two signals at its inputs. Thus, because the two inputs to the power combiner 110 are a sample of the original input signal and a sample of the input signal plus the distortion introduced by the main amplifier 102, the power combiner 110 output is the distortion of the main amplifier 102. The first loop modulator 103 may be used to adjust the amplitude and phase of the signal routed to the main amplifier 102 to minimize the amount of input signal (carrier) at the power combiner 110 output.
The output of the power combiner 110 is coupled to a second loop modulator 111. The second loop modulator 111 adjusts the amplitude to a level to match the distortion level out of the main amplifier 102, and the phase is adjusted to be 180xc2x0 out of phase with the main amplifier 102 output, when combined in a distortion canceling directional coupler 114. The output of the second loop modulator 111 is routed to an error amplifier 112. The error amplifier 112 buffers the output of the second loop modulator 111. The error amplifier 112 output is combined with the main amplifier 102 output from the second loop delay 108 in a distortion canceling directional coupler 114. The delay experienced by the signal passing through the second loop delay 108 is selected to match the delay experienced by the signal passing through the power combiner 110 and the error amplifier 112. Thus, the output of the distortion canceling directional coupler 114 is a substantially non-distorted replica of the input signal.
Typical problems in conventional feedforward amplifiers, as illustrated in FIG. 1, are that the amplifier performance is dependent on component characteristics and tolerances that affect the gain and phase of the device. In particular, proper operation of the feedforward amplifier requires that proper gain and phase be maintained for the feedforward signal to effectively cancel the distortion present in the main amplifier output. Thus, variations due to, for example, component age, component tolerance, or operating conditions such as temperature, power supply voltage, operating frequency, and humidity, may adversely affect the performance of the feedforward amplifier. In addition, there is no feedback to allow adjustments to correct for errors introduced by the components.
FIG. 2 is a block diagram of an improved feedforward amplifier 115 incorporating a pilot signal. Incorporating a pilot signal into the conventional feedforward amplifier 100 addresses several of the problems associated with the conventional feedforward amplifier 100. In the improved feedforward amplifier 115, a test signal, or pilot signal, is inserted into the signal path before the first loop modulator 103 and the main amplifier 102 via a pilot signal directional coupler 116. In the pilot signal directional coupler 116 the pilot signal is mixed with the main portion of the input signal before delivery to the first loop modulator 103 and the main amplifier 102.
The main amplifier 102 output, containing the pilot signal, is routed through the second loop delay 108 and is then sampled by an output sampling directional coupler 118. The output sampling directional coupler is connected to a pilot detector 120. The pilot detector 120 selectively detects the pilot signal present in the improved feed forward amplifier 115 output. For example, the detector may be a bandpass filter with a center frequency at the pilot signal frequency followed by an envelope detector. The output of the pilot detector 120 represents the amplitude of the pilot signal present in the output of the improved feedforward amplifier 115. The pilot detector 120 output is routed to a second loop controller 122.
As in the conventional feedforward amplifier illustrated in FIG. 1, the output of the main amplifier 102 is sampled by the first loop directional coupler 106 and routed to the power combiner 110. In the improved feedforward amplifier 115, the two inputs to the power combiner 110 are a sample of the main amplifier 102 output, with distortion and the pilot signal, and a sample of the original input signal. Thus the power combiner 110 output is the distortion of the main amplifier 102 plus the pilot signal. The first loop delay 104 is selected to match the delay introduced by the pilot signal directional coupler 116, the main amplifier 102, and the first loop directional coupler 106. As described above the first loop modulator 103 may be used to adjust the amplitude and phase of the signal routed to the main amplifier 102 to minimize the distortion of the main amplifier 102.
The output of the power combiner 110 is routed to a second loop modulator 124. The second loop modulator 124 modifies the amplitude and phase of the output of the power combiner 110 as commanded by the second loop controller 122. The output of the second loop modulator 124 is routed to the error amplifier 112. As discussed in relation to the conventional feedforward amplifier 100 illustrated in FIG. 1, the error amplifier output is routed to a distortion canceling directional coupler 114 and is combined with the main amplifier 102 output.
The second loop controller 122 commands the second loop modulator 124 to adjust the amplitude and phase of the power combiner 110 output to minimize the amplitude of the pilot signal present in the improved feedforward amplifier 115 output, as measured by the detector 120. The pilot signal behaves the same as the distortion introduced by the main amplifier 102. Thus, minimization of the pilot signal present in the improved feedforward amplifier 115 output also minimizes the presence of distortion introduced by the main amplifier 102 in the amplifier output.
The use of a pilot signal in a feedforward amplifier addresses some of the problems of conventional feedforward amplifiers by providing a feedback mechanism to compensate for errors introduced by component variations. However, several disadvantages still exist because of the pilot signal used. For example, if a single tone, or frequency hopping, pilot signal is used then the feedforward amplifier optimization is based at the particular frequency of the pilot tone, not the particular frequency bands being amplified by the feedforward amplifier. If a broadband signal, such as a noise or spread spectrum signal, is used as a pilot signal, then the feedforward amplifier optimization is based on the entire frequency band of the broadband signal, not the particular frequency bands being amplified by the feedforward amplifier. In both these examples the optimization of the feedforward correction is not optimal because it is not based on the particular frequency bands that are being amplified by the feedforward amplifier.
From the discussion above, it should be apparent that there is a need for a system that can provide linearization of a feedforward amplifier that is adapted in response to the particular frequency bands being amplified by the feedforward amplifier.
A method and apparatus for reducing distortion in the output of a feedforward amplifier includes a first and second loop. The first loop includes a modulation source, and is configured to accept an input signal. The input signal is then modulated with the modulation source. The first loop outputs a modulated signal.
The second loop is configured to accept the modulated signal output from the first loop. The second loop includes detection of the presence of the modulation source in the modulated signal. An error signal is generated in response to the detected modulation source. The error signal is summed with the modulated signal to minimize the presence of the modulation source in the modulated signal.